The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.

/pdf/gas-sensors-based-on-conducting-polymers-4l6i49715q.pdf

Gas Sensors Based on Conducting Polymers

Computational methods for the analysis of chemical sensor array data from volatile analytes.

Polyaniline as a new sensitive layer for gas sensors

Thin films of polyaniline have been deposited by spinning, evaporation and by the Langmuir-Blodgett technique. The films are shown to possess slightly different in-plane electrical conductivities, reflecting differences in their chemical structure and layer morphology. The conductivity is found to depend on the gas ambient. All types of polyaniline films are sensitive to H2S and NOx at concentrations down to 4 ppm. However, only spun and evaporated films are responsive to SO2.

Polyaniline thin films for gas sensing

Thin, strongly adhering films of single-walled carbon nanotube bundles (SWNT) on flexible substrates such as poly(ethyleneterephthalate) (PET) were used for vapour sensing (hexane, toluene, acetone, chloroform, acetonitrile, methanol, water, etc.). These sensors are extremely easy to fabricate using the line patterning method. For example, ‘4-probe’ sensor patterns are drawn on a computer and then printed on overhead transparency (PET) sheets. These PET patterns were coated with films of electronically conductive SWNT bundles (1–2 μm thick) by dip-coating in aqueous surfactant-supported dispersions and mounted in glass chambers equipped for vapour sensing. Experiments conducted under saturated vapour conditions in air showed sensor responses that correlated well with solvent polarity [ET(30) scale]. Similar results were obtained under controlled vapour conditions (no air) at 10,000 ppm. Control experiments using films of carbon black on PET (Aquadag-E®), also prepared by the line patterning method, showed very little response to vapours under identical experimental conditions. The sensors are very flexible, e.g., they can be bent to diameters as small as 10 mm without significantly compromising sensor function.

/pdf/flexible-vapour-sensors-using-single-walled-carbon-nanotubes-3g589w96jy.pdf

Flexible vapour sensors using single walled carbon nanotubes

Organic thin-film sensors bases on the thermal evaporation and dip-coating of polyaniline, and on the Langmuir-Blodgett deposition of a vanadium porphyrin, have been fabricated. The d.c. electrical resistance of the individual elements are found to exhibit different changes on exposure to simple vapours (water, proano, ethyl acetate and acetone). These data have been used successfully to train an artificial neural network, based on a back-propagation technique, to recognized two of the vapours.

Vapour recognition using organic films and artificial neural networks

A hybrid phthalocyanine/silicon field-effect transistor sensor for NO2

A polyaniline/sllicon hybrid field effect transistor humidity sensor

Solvent-cast polyaniline films have been deposited by spin coating and photolithographic techniques used to pattern the films to a resolution of 15–20 μm. This approach is then used successfully to deposit the polymer on the gate areas of an array of silicon field effect transistors.

A photolithographic technique for patterning spin-coated polyaniline films

We report on the successful fabrication of an array of charge-flow transistors incorporating polyaniline films. The electrical d.c. characteristics have been found to be similar to those of conventional metal-oxide-semiconductor field-effect transistors. However, on application of a gate voltage, Vgs, greater than the threshold voltage, a delay is observed in the response of the drain current. This is attributable to the time taken for the resistive polyaniline layer to charge up to Vgs. These delay characteristics have been found to be dependent upon the ambient, suggesting that the charge-flow transistor device can be used as an effective gas sensor. Results are presented of studies using NOx and SO2, showing that the device can detect reversibly gas concentrations down to 1 ppm.

P.S. Barker

Papers

Vapour recognition using organic films and artificial neural networks

A hybrid phthalocyanine/silicon field-effect transistor sensor for NO2

A polyaniline/sllicon hybrid field effect transistor humidity sensor

A photolithographic technique for patterning spin-coated polyaniline films

Gas sensing using a charge-flow transistor